Conventionally, unstretched films and stretched films made of aliphatic polyamide typified by nylon 6 and nylon 66 are excellent in impact resistance and pin-hole resistance, and are widely used for various packaging materials.
In liquid-filled packages for soups and seasoning agents, pin-hole-resistant stretched polyamide-based resin films are widely used which are further softened by mixing various elastomers with aliphatic polyamide in a single-layer configuration for improving pin-hole resistance and impact resistance.
In the conventional pin-hole-resistant films, films in which a polyolefin-based elastomer is mixed with aliphatic polyamide have the problem that although pin-hole resistance and impact resistance are satisfactory at normal temperature, but pin-hole resistance and impact resistance are degraded under a low-temperature environment, so that pin-holes easily occur due to bending fatigue during transportation of commercial products. The occurrence of pin-holes in packaging materials of commercial products may cause contamination by leakage of contents, decay of contents and generation of fungus, leading to a reduction in commercial value.
For solving such a problem, for example, Patent Documents 1 and 2 describe films in which a polyamide-based elastomer is mixed with aliphatic polyamide. The films described in Patent Documents 1 and 2 have satisfactory pin-hole resistance and impact resistance under a low-temperature environment and are hard to cause pin-holes due to bending fatigue even under a low-temperature environment.
However, the films described in Patent Documents 1 and 2 have the problem that the film has a reduced laminate strength when a polyethylene film is dry-laminated for imparting heat seal performance.
Since the dispersion diameter of a polyamide elastomer in aliphatic polyamide is large, impact when a bag forming article is caused to fall successively propagates to polyamide elastomer particles having a large particle size dispersed in a polyamide film, so that the film tends to easily tear in a thickness direction. Therefore, large liquid product bags using a pin-hole resistant polyamide film having a polyamide elastomer added are subject to falling bag breakage.
Further, in the case of bag forming articles of a polyamide-based film/polyethylene resin configuration in which the thickness of a polyethylene resin layer for imparting heat seal performance is increased or a harder polyethylene resin is selected for the purpose of imparting stiffness to the bag forming article in order to improve handling characteristics during bag formation processing and workability at the time of filling the bag with contents, the bag forming article has increased stiffness, but tends to get pin-holes. Therefore, polyamide-based films to be used for these bag forming articles are required to have further enhanced pin-hole resistance.
However, when the content of a polyamide elastomer in the film is increased for enhancing pin-hole resistance, the problem arises that the laminate strength is further reduced and transparency is also deteriorated.
Demand is also growing for large liquid product bags for business use which have a volume of not less than 1 liter, and bag-in-boxes having a packaging form in which large plastic bags are included in a corrugated carton to store and transport products. Polyamide films to be used for these bags considerably require pin-hole resistance and impact resistance, and require strong falling bag breakage resistance and strong cohesive fracture strength in a film thickness direction in order to package a larger-volume liquid product.
Further, there exist many commercial products in the market, the quality assurance period of which is extended by boiling a packaging bag filled with contents in hot water to be sterilized, and these bags are required to retain adhesive strength in hot water, retain adhesive strength under water attaching conditions, and cause no external appearance deterioration such as whitening in water.
Thus, current pin-hole-resistant polyamide-based films cannot sufficiently satisfy quality that is required in the market.